Open cavities are usually used in gyromonotron oscillator and gyroklystron. Because open cavities can efficiently obtain EM field from electron beam. The main difference between open cavities and enclosed cavities is that open cavities lack obvious bound. Therefore, we can’t get an analytical solution of EM wave in open cavities. We must use numerical simulation to know the property of open cavities. In this thesis, we will start from basic theory and model of time domain and spectral domain. Then, we will discuss the influence of parameter of structure on quality factor and resonant frequency by numerical simulation. Finally, we will try to simulate a new structure of open cavities and make reasonable interpretation of property of new structure.